Effect of Dilute Acid on Hydraulic Fracturing of Carbonate Rich Shales – Modeling Study

Abstract In Teklu et al. (2017a, and 2017b) matrix and fracture permeability of carbonate rich and clay rich shale cores were measured before and after exposing the core samples to spontaneous dilute acid imbibition. In this study, effect of brine and dilute acid imbibition on long term production of shale gas reservoir is investigated using numerical modeling approach. Experimental observations show that: (1) matrix permeability of clay rich shale samples are usually impaired / damaged by dilute HCl imbibition. This impairment is due to both the clay swelling and related issue of brine (within the dilute HCl solution) and rock-acid reaction. (2) Matrix permeability of calcite rich shales improved with dilute acid imbibition. (3) Effective fracture permeability of un-propped (natural fracture) calcite rich and clay rich shales are reduced by dilute acid imbibition, this is because of "rock softening" and "etching/smoothing" of fracture roughness on the "fracture faces". (4) Propped permeability of clay and carbonate rich shales decrease with brine imbibition, and permeability further decreased after dilute acid imbibition. The further decrease in permeability and porosity after acid imbibition is caused by three phenomena: (a) rock weakening by rock-acid reaction, (b) further proppant embedment during repeated testes vs. stress, and (c) scale precipitation. These observations are similar to results reported in Teklu et al. (2017a and 2017b). Modeling results show that: (1) For carbonate rich shale reservoirs, despite dilute acid injection/imbibition/fracturing can cause considerable damage to propped fractures and natural fractures (mainly due to rock softening/weakening, fracture roughness damage, and proppant embedment), it can still lead to considerable production improvement This is mainly caused by the matrix permeability improvement which leads to improvement in stimulated reservoir volume. (2) For clay rich shale reservoirs, both brine (slickwater) and dilute acid imbibition/injection/fracturing reduces hydrocarbon production, due to damages caused in to propped fractures, natural fractures, and matrix. Therefore, dilute acid injection or imbibition is recommended pre, post, or during hydraulic fracturing of carbonate rich shale reservoirs (but not for clay rich shale reservoirs) Moreover, slickwater fracturing in clay rich shale reservoirs can create considerable formation damage and decrease in hydrocarbon production, hence, dry gas (such as N2, CO2, LNG, etc.) might be an alternative solution hydraulic fracturing of in clay rich shale reservoirs.